Sensor and methods for measuring select components in moving sheet products

Abstract

A compact, long lasting sensor for measuring components such as moisture in moving sheets including paper in a papermaking apparatus employs light sources that produce radiation within defined wavelength regions of interest and the light sources are modulated at high frequencies using non-mechanical techniques. A single detector with various radiation sources can measure at all frequencies while keeping information separated. Superluminescent light emitting diode or laser diode light sources can be directly electrically modulates for improved noise rejection. These higher power and bright light sources afford excellent fiber optic launch efficiency and permits the sensor to be scanned at much faster rates over the paper being monitored.

1. A sensor for measuring at least one selected component in a composition, comprising:
 at least one light source operable to generate light having a desired wavelength range; 
 a controller operable to modulate the at least one light source without mechanically modulating the at least one light source; 
 a detector operable to receive and measure light that emerges from the composition; 
 an optical head comprising first optics operable to direct the light from the at least one light source to the composition and second optics operable to direct the light that emerges from the composition to the detector, wherein the first and second optics comprise first and second mirrors, respectively, and wherein the light from the at least one light source is transmitted through an optical fiber to the optical head; and 
 a signal analyzer operable to analyze measurements of the light that emerges from the composition and to correct for bend loss in the optical fiber. 
 
   
   
     2. The sensor of  claim 1 , wherein the at least one light source comprises at least one of: a superluminescent light-emitting diode and a laser diode. 
   
   
     3. The sensor of  claim 1 , wherein the controller is operable to directly modulate a drive current of the at least one light source. 
   
   
     4. The sensor of  claim 1 , wherein the controller is operable to employ one or more external electro-optical modulators. 
   
   
     5. The sensor of  claim 1 , wherein the controller is operable to employ one or more external acousto-optical modulators. 
   
   
     6. The sensor of  claim 1 , wherein the controller is operable to modulate the at least one light source at a rate of at least 5 kHz. 
   
   
     7. The sensor of  claim 1 , wherein the at least one light source comprises a light emitting device coupled to the optical fiber so that light from the light emitting device is transmitted through the optical fiber to the optical head. 
   
   
     8. The sensor of  claim 7 , wherein the optical head further comprises a coupler operable to be coupled to the optical fiber. 
   
   
     9. The sensor of  claim 1 , wherein:
 the optical head is configured to move; and 
 further comprising a cable take-up mechanism configured to manage the optical fiber to preserve an overall bend length and radius of the optical fiber. 
 
   
   
     10. The sensor of  claim 1 , wherein:
 the detector is operable to generate a signal that is proportional to an intensity of the light received; and 
 the sensor further comprises a filter operable to filter noise from the signal. 
 
   
   
     11. The sensor of  claim 10 , wherein the filter comprises lockin amplifiers operable to amplify a modulated signal and to convert the modulated signal to a proportionate DC level signal while simultaneously suppressing the noise with a low-pass filter. 
   
   
     12. The sensor of  claim 11 , wherein the lockin amplifiers are operable to remove background noise. 
   
   
     13. The sensor of  claim 1 , wherein no optical filter is used to generate the light having the desired wavelength range. 
   
   
     14. A sensor for measuring at least one selected component in a composition, comprising:
 a first light source operable to generate first light having a first wavelength region that is sensitive to the at least one selected component; 
 a first controller operable to modulate the first light source without mechanically modulating the first light source; 
 a second light source operable to generate second light having a second wavelength region that has a different sensitivity to the at least one selected component; 
 a second controller operable to modulate the second light source without mechanically modulating the second light source; 
 a detector operable to receive and measure third light that emerges from the composition, the third light based on the first light and the second light; and 
 a signal analyzer operable to analyze measurements of the third light, wherein the first light and the second light are directed at the composition through an optical fiber, and wherein the signal analyzer is operable to use measurements associated with a portion of the third light that is based on the second light to correct for bend loss in the optical fiber. 
 
   
   
     15. The sensor of  claim 14 , wherein:
 the first controller is operable to modulate the first light source at a first frequency to generate a first modulated light signal; and 
 the second controller is operable to modulate the second light source at a second frequency to generate a second modulated light signal, wherein the first frequency is different from the second frequency. 
 
   
   
     16. The sensor of  claim 15 , wherein the first modulated light signal is multiplexed with the second modulated light signal for transmission through the optical fiber. 
   
   
     17. The sensor of  claim 16 , wherein frequency division multiplexing is employed to multiplex the first and second modulated light signals. 
   
   
     18. The sensor of  claim 15 , wherein the detector comprises a single detector and a single pre-amplifier operable to detect light having the first frequency and the second frequency. 
   
   
     19. The sensor of  claim 14 , wherein:
 the first light source comprises a first light emitting device coupled to the optical fiber so that the first light from the first light emitting device is transmitted through the optical fiber; and 
 the second light source comprises a second light emitting device coupled to the optical fiber so that the second light from the second light emitting device is transmitted through the optical fiber. 
 
   
   
     20. An apparatus for measuring at least one selected component in a composition, comprising:
 at least one light source operable to generate light having a desired wavelength range; 
 a splitter operable to split the light from the at least one light source and to deliver the light to a plurality of positions on the composition; 
 a controller operable to modulate the at least one light source without mechanically modulating the at least one light source and to adjust operation of the at least one light source to account for temperature-dependent wavelength shifting associated with variations in a temperature of the composition; and 
 a plurality of detectors operable to receive light that emerges from the composition at the plurality of positions. 
 
   
   
     21. The apparatus of  claim 20 , wherein:
 the at least one light source comprises:
 a first light source operable to generate first light having a first wavelength region that is sensitive to the component; and 
 a second light source operable to generate second light having a second wavelength region that has a different sensitivity to the component; and 
 
 the controller comprises:
 a first controller operable to modulate the first light source without mechanically modulating the first light source; and 
 a second controller operable to modulate the second light source without mechanically modulating the second light source. 
 
 
   
   
     22. The apparatus of  claim 21 , wherein:
 the first controller is operable to modulate the first light source at a first frequency to generate a first modulated light signal; and 
 the second controller is operable to modulate the second light source at a second frequency to generate a second modulated light signal, wherein the first frequency is different from the second frequency. 
 
   
   
     23. The apparatus of  claim 22 , wherein the first modulated light signal is multiplexed with the second modulated light signal for transmission through an optical fiber. 
   
   
     24. The apparatus of  claim 23 , wherein frequency division multiplexing is employed to multiplex the first and second modulated light signals. 
   
   
     25. A method for sensing a substance in a composition, comprising the steps of:
 irradiating the composition with radiation including wavelengths in at least first and second separate wavelength regions, wherein the radiation is provided by one or more light sources that are modulating non-mechanically, and wherein the radiation in the first wavelength region is more sensitive to the substance in the composition than the radiation in the second wavelength region; 
 detecting an amount of radiation that emerges from the composition in the first and second separate wavelength regions; and 
 adjusting operation of the one or more light sources to account for temperature-dependent wavelength shifting associated with variations in a temperature of the composition. 
 
   
   
     26. The method of  claim 25 , wherein the irradiating step comprises irradiating the composition with a first modulated light signal that is modulated at a first frequency and a second modulated light signal that is modulated at a second frequency, the second frequency different from the first frequency. 
   
   
     27. The method of  claim 26 , wherein the first modulated light signal is multiplexed with the second modulated light signal for transmission through an optical fiber. 
   
   
     28. The method of  claim 27 , wherein frequency division multiplexing is employed to multiplex the first and second modulated light signals. 
   
   
     29. The method of  claim 25 , further comprising the step of computing an amount of the substance in the composition based upon the detected amount of radiation in the first and second wavelength regions. 
   
   
     30. The method of  claim 25 , wherein the composition is a flat sheet product that comprises at least one of: paper and plastic. 
   
   
     31. The method of  claim 25 , wherein:
 the composition comprises paper; and 
 a level of water in the paper is computed.